Personal Data Cards (PDC), also known as "Smart Cards," are devices that include one or more microelectronic chips embedded in a piece of plastic about the size of a conventional credit card. Typically, the chips include a microprocessor to perform computing operations and some form of memory, such as an EEPROM, for storage. Such cards may be used, for example, in a manner similar to a "debit" card for long distance telephone calls, retail store purchases and automatic banking machines. Other uses include personal identification and general data storage which may be modified from time to time by the card holder or the card issuer. Background material for such cards can be found in an article entitled "Smart Credit Cards: the answer to cashless shopping" published in the February, 1984 issue of IEEE Spectrum at pages 43-49; and in an article entitled "Smart Cards" published in the November, 1985 issue of Scientific American at pages 152-159.
Power transfer to the PDC is conventionally achieved via metallic contacts which, unfortunately, are subject to oxidation, corrosion, and the deposit of surface contaminants that may increase ohmic resistance in one situation, or create a short circuit between adjacent contacts in another. Such metallic contacts need to be electrically and mechanically rugged to provide reliable results over their expected lifetime.
One solution to this general problem is disclosed in U.S. Pat. No. 4,480,178 issued Oct. 30, 1984 to R. R. Miller II, et al for a "Tuning Arrangement for Interfacing Credit Card-Like Device to a Reader System," assigned to the assignee hereof and incorporated herein by reference. The reference discloses an arrangement that provides operating power to the PDC through a capacitive interface. A variable inductor automatically tunes a power transfer circuit to resonance and thereby maximizes power transfer to the card. Unfortunately, the size of the capacitor plates limits the amount of power that can be transferred to the card.
An improved technique for transferring power to the PDC is disclosed in U.S. Pat. No. 4,692,604 issued Sept. 8, 1987 to Billings for a "Flexible Inductor," assigned to the assignee hereof and incorporated herein by reference. This application discloses a card having a flexible coil and a flexible ferromagnetic member which, when inserted into an associated card reader/writer unit, inductively couples to a transformer primary so that the coil in the PDC operates as a transformer secondary and, in that role, receives electrical power from the reader/writer unit. No provision, however, is made in such an arrangement to regulate the amount of power that is delivered to the card. The proper positioning of the PDC in the reader/writer unit is of some significance in this regard; equipment tolerances and variation in air gaps among different PDCs may cause too little or too much power to be transferred to the card. Too little power would not activate the circuitry while too much power would damage it. Another general concern is the need to determine whether a conventional credit card or a PDC is being inserted into the reader/writer unit without requiring the card holder to enter such information.
Accordingly, it is an object of the present invention to provide a card reader/writer unit with the ability to distinguish the type of card (conventional or PDC) that is inserted therein.
It is another object of the invention to transfer a predetermined amount of power to a PDC, inserted into a card reader/writer unit, regardless of card warpage or improper alignment between the card and the unit.